EP0519796B1 - Multiplexer for inserting digital packets with different priority levels in a transmission channel - Google Patents

Abstract

The device, applicable in particular to the construction of the digital multiplex of an X-MAC/PACKET signal, enables digital packets provided by various sources to be inserted into the channel. The device comprises a multiplexer (14) linked to the transmission channel. Each source (18) comprises means for formulating, for each packet, an order of priority of insertion as a function of the type of packet, of the number of packets in reserve and of the insertion rules related to the type of packet to be transmitted. The multiplexer (14) reads the order of priority of the packets presented by the sources and selects the packet having the highest priority. <IMAGE>

Description

The present invention relates to a device for inserting digital packets supplied by different sources into a transmission channel having an average bit rate at least equal to the sum of the average bit rates provided by the sources. It finds a particularly important application in the insertion of packets to constitute the digital multiplex of a television signal belonging to the X-MAC / PACKET family.

In an X-MAC / PACKET signal, the bit stream intended for the transmission and broadcasting of sound and data is divided into packets of fixed length, each having a 23-bit address, making it possible to identify the origin of the packet, and a useful part of 728 bits. The channel has a flow rate of constant average value (which can, depending on the case, range from 2050 to 24,600 packets per second) but of variable instantaneous value. The input member of the output channel, constituted for example by the time multiplexer for forming the multiplex intended to be addressed to the modulator performs packet requests as and when it becomes available.

Each source has its own flow characteristics and insertion rules in the output channel. The order of insertion of packets into the output channel must take these elements into account.

In the case, for example, of a D2-MAC / PACKET system with conditional access according to the "EUROCRYPT" standard, the digital channel may have to transmit one or more sound channels each requiring a capacity of 250 to 1778 packets per second and whose offset from the image must remain low (original sound and dubbing for example), one or more high-fidelity sound channels requiring a higher speed than the previous one, a service identification channel consisting of packets having the address zero, and packets representing the access title control messages or ECM (Entitlement Checking Messages) and the access title management messages or EMM (Entitlement Management Messages). In addition, the digital channel must transmit possible teletext signals in packets.

We see that the sources have bit rates which vary from source to source and insertion rules which can also vary.

There are already several insertion techniques.

The one described in the document "The EBUC-MAC / PAQUET system for direct broadcasting by satellite", by Mertens et al, IEEE International Conference on Communications, ICC 84, vol. 1, p. 1-9 North-Holland, New York, involves presenting data from various sources to a multiplexer with input buffers equal in number to the sources. Whenever the multiplexer is ready to packet, it examines the buffers in a priority order which depends on the service continuity needs and chooses the data in a memory which contains enough information to constitute a whole packet. .

Document FR-A-2 629 972 describes a priority management device having two cascading queues; the output of the second is connected to an input of a multiplexer; the input of the first is connected to a comparator and to the other input of the multiplexer.

The invention aims to provide an insertion device which meets the requirements of practice better than those previously known, in particular in that it makes it possible to add additional sources without appreciable additional work for the packet multiplexer, therefore without necessity. modification of the latter, all sources having the same general constitution and being achievable as an integrated circuit.

To this end, the invention proposes in particular a device for inserting digital packets in a transmission channel according to claim 1.

In such a device, the packet storage function is reported in the sources.

When the average bit rate of the channel is significantly higher than the average total bit rate of the sources and the variations of instantaneous bit rate available represent only a small fraction of the average bit rate, each source can be provided to fix the order of priority of the packet to be sent. once for all. When, on the contrary, the average bit rate of the channel is little higher than the sum of the average bit rates, it may be preferable to plan the sources so that they give the packets an increasing priority as a function of the waiting time.

In the particular case, mentioned above, of an insertion device for a D2-MAC / PACK modulator with conditional access, it is possible to significantly delay the access control messages, which are only repeated at intervals of the order of 500 ms, with modification of the control words every 10 seconds, with an advance on the moment of modification of the control word, and even more the messages of management of the titles and access rights that suffices transmit at intervals low enough that subscribers do not experience undue delay.

The sources connected in cascade can attack the input member via a selection interface between several input channels in parallel.

• la figure 1 est un schéma synoptique de principe montrant un dispositif d'insertion selon un mode particulier de réalisation ;
• la figure 2 montre une variante de réalisation ;
• la figure 3 montre une organisation possible des priorités, dans le cas d'un multiplex D2.

The invention will be better understood on reading the following description of a particular embodiment given by way of non-limiting example. The description refers to the accompanying drawings, in which:

• Figure 1 is a block diagram showing an insertion device according to a particular embodiment;
• Figure 2 shows an alternative embodiment;
• FIG. 3 shows a possible organization of the priorities, in the case of a D2 multiplex.

The insertion device shown in FIG. 1 is intended to supply the input device 10 of an output channel, the packets intended to constitute the digital part, coded in duo-binary form, of a time band multiplex basic formed by a multiplexer 12. For this, the input member 10 is provided, in a conventional manner, to send requests D packets, depending on availability, and distribute the packets P it receives in bursts.

The embodiment shown in FIG. 1 comprises several input channels in parallel and a packet multiplexing selector 14 making it possible to orient, towards the member 10, the packet having the highest priority. For this it comprises a selector proper 15, memories 16 ₁ , 16 ₂ , ..., 16 _n each intended to store an order of priority corresponding to a packet available from a respective channel 1, 2, .. n and a circuit 17 for accounting for packets in reserve in the memories and for memorizing their type.

The multiplexing selector is intended to select the source which corresponds to the highest priority order and to transfer a packet (or several packets in determined number) from the selected input channel to the output channel.

• en fonction du type de paquets, suivant un critère fixe pour la source,
• du nombre de paquets en attente, l'ordre de priorité s'accroissant au fur et à mesure du remplissage de la mémoire de la source,
• de règles d'insertion prédéterminées, éventuellement en tenant compte d'un signal d'acquittement provenant du multiplexeur 12 formant le multiplex en bande de base.

Channels 1 to n-1 shown have a single source. The source 18 ₁ for example is provided for grouping the data it receives, in packets all having the same address and defining a priority. The source may in particular include a buffer memory 19 for accumulating incoming data and means for formatting this data into packets having a header and assigning a priority to packets. This assignment can be done:

• depending on the type of packets, according to a fixed criterion for the source,
• the number of pending packets, the order of priority increasing as the source memory is filled,
• predetermined insertion rules, possibly taking into account an acknowledgment signal from the multiplexer 12 forming the baseband multiplex.

All sources can have the same constitution, of the kind described above.

But some sources may have a particular function.

For example, in the illustrated case, the source 18 _n-1 is constituted by a source of stuffing packets, having the lowest priority, to which the address 1023 is assigned.

Channel n has a particular constitution, in accordance with the invention. The downstream source 18 _n of this channel constitutes a multiplexing interface, allowing the packet multiplexing selector 14 to process, without hardware or software complication, several sources. For this, the source 18 _n comprises a routing circuit 20 providing on its output the highest priority among the two priorities applied to its input and each of the additional sources 18 _{n + 1} , 18 _{n + 2} , ..., up to 'the penultimate included, has the same constitution. Thus the only priority which is presented to the packet multiplexing selector 14 is the highest among those of all the additional sources. A possible constitution of the sources will be described later with reference to Figure 2.

The operation of the device follows immediately from the foregoing description: when a packet request D is sent to the packet multiplexing selector 14, that determines the source which has the highest priority and sends a request to it, D1 for example. In response, the interrogated source sends a packet, P1 by example, towards the exit.

The multiplexing device shown in Figure 2 (where the elements corresponding to those in Figure 1 have the same reference number) differs from the previous in that it comprises a single input channel. Consequently, the multiplexing selector 14 is very simplified.

All sources can have the same constitution.

Each of them, except for the most upstream source 18 _{n + j} , has the same function. The source 18 _n for example comprises a packetizing circuit 19 and a switching circuit 20. Each switching circuit can be viewed as having a priority comparator 22 and two switches 24 and 26 controlled by the comparator 22. This the latter receives a local priority order indication prepared by the circuit 19 and an upstream priority order indication coming from the upstream source. It transmits the highest priority downstream and directs the switch 24, upwards (in the case of FIG. 2) if the local priority is equal to or greater than the priority upstream, downwards otherwise . The priority retained downstream is communicated to a transmission authorization generator 28 belonging to the selector 14. The latter sends, in response to a packet request, an authorization which is directed to the appropriate circuit 19 by the first oriented switch 24 up that is met. This switch causes the transmission, by the circuit 19, of a packet which arrives at the selector 14 by the switches 20 of the source which transmits and of those placed downstream. Multiplexer 12 may be provided to return an ACK acknowledgment signal after a burst is inserted.

• paquets de bourrage,
• messages de gestion des titres d'accès,
• messages de contrôle des titres d'accès,
• voie zéro (dont l'ordre de priorité peut être modulé en fonction du nombre de paquets à émettre sur cette voie),
• voie de données,
• voie son à débit élevé, lorsque la source n'a pas commencé à émettre un groupe constitué d'un nombre déterminé de paquets en cours,
• voie son à débit élevé, lorsqu'une émission est en cours,
• voie son à faible débit.

In the already envisaged case of an insertion device for a D2-MAC modulator, the order of increasing priority may be as follows:

• stuffing packs,
• access title management messages,
• access title control messages,
• channel zero (the order of priority of which can be varied depending on the number of packets to be sent on this channel),
• data channel,
• high-speed sound channel, when the source has not started transmitting a group consisting of a determined number of packets in progress,
• high speed sound channel, when a transmission is in progress,
• sound channel at low speed.

The last two priorities can be increased or decreased depending on the number of packets to be transmitted.

• 10 pour les messages de gestion destinés à un abonné unique, à un groupe partagé et à l'ensemble de l'audience (EMM-U, EMM-S et EMM-G) ;
• 20 pour les messages de gestion destinés aux collectivités (EMM-C) ;
• 30 pour les messages de contrôle des titres d'accès (ECM) ;
• 40 pour la voie zéro ;
• 50 pour les blocs d'interprétation du son BI ;
• 60 pour les blocs d'interprétation indiquant les changements de nature du signal BIT.

FIG. 3 shows, by way of example, the priority levels which can be assigned to the different packets, during insertion into a D2 MAC modulator implementing the conditional access "EUROCRYPT" defined by the publication "System of conditional access for the MAC / PACKAGE family: EUROCRYPT ", March 1989, Ministry of Posts, Telecommunications and Space and others. Fixed and staged priorities are assigned to the access title management messaging (EMM), to the access title control messaging (ECM), to the zero channel and to the BI blocks for interpreting the sound channels. In the example shown, the increasing priorities are represented by:

• 10 for management messages intended for a single subscriber, a shared group and the entire audience (EMM-U, EMM-S and EMM-G);
• 20 for management messages intended for communities (EMM-C);
• 30 for access title control messages (ECM);
• 40 for the zero channel;
• 50 for the BI sound interpretation blocks;
• 60 for the interpretation blocks indicating the changes in the nature of the BIT signal.

As a variant, the priority of the zero channel can be modulated as a function of the number of packets to be transmitted. A counter can be provided to memorize the number of pending packets and to jump the priority from 40 to a value equal to or greater than 50 when this number exceeds a determined value.

The priority to assign to sound channels must take into account that they can belong to several categories, obeying different distribution laws. A first law, or "90 byte law per block" corresponds to the transmission of sound in compressed form with first level protection or to linear coded transmission with second level protection.

The second law, at 120 bytes per block and distribution at the rate of three blocks for four packets, corresponds to a linear coding with first level protection or to a compressed coding with second level protection. The groups of four packets constituting three blocks for the 120 byte per block laws must not be interleaved with packets from another channel of the same type.

• Pour chaque voie son avec loi à 90 octets, la priorité minimum est de 80 et elle est augmentée d'une unité pour chaque paquet en réserve : la priorité passe ainsi à 84 pour cinq paquets en réserve. En général, on prévoiera une valeur maximale, par exemple de 89.
• A chaque voie son ayant une loi à 120 octets et trois blocs pour quatre paquets, une priorité de base plus faible est affectée, par exemple de 70. Elle peut être accrue d'une unité pour chaque paquet en attente, avec un maximum de 79, aussi longtemps qu'il n'y a pas début d'émission d'un groupe de trois blocs. Dès que commence une telle émission, la priorité des paquets suivants du même groupe est augmentée d'un nombre d'unités tel que la nouvelle priorité soit supérieure à celle de toutes les autres voies ayant une loi à 120 octets par bloc. L'augmentation peut par exemple être effectuée par incrémentation de la priorité d'un nombre égal à la différence entre 10 et 1 pour chaque paquet déjà émis.

One solution to both take this imperative into account and avoid over-favoring one path over another is to adopt the following priority rule.

• For each channel with 90-byte law, the minimum priority is 80 and it is increased by one for each packet in reserve: the priority thus goes to 84 for five packets in reserve. In general, a maximum value will be provided, for example 89.
• Each channel has a 120 byte law and three blocks for four packets, a lower basic priority is assigned, for example 70. It can be increased by one unit for each pending packet, with a maximum of 79 , as long as there is no start of emission of a group of three blocks. As soon as a such transmission, the priority of the following packets of the same group is increased by a number of units such that the new priority is higher than that of all the other channels having a law with 120 bytes per block. The increase can for example be carried out by incrementing the priority by a number equal to the difference between 10 and 1 for each packet already transmitted.

• 83 pour quatre paquets en réserve ;
• 82 pour trois paquets en réserve ;
• 81 pour deux paquets en réserve.

So, for example, priority goes to:

• 83 for four packages in reserve;
• 82 for three packages in reserve;
• 81 for two packages in reserve.

Once the four packets have been sent, the corresponding sound source returns to its normal priority rank, which corresponds to 70 when there is only one packet waiting.

The lowest priority is assigned to jam packets.

Such an integration strategy is extremely flexible and makes it easy to introduce sources by assigning intermediate priorities to them.

Claims (7)

1. Multiplexing device for insertion of digital packets, supplied by a plurality of different sources, in a same transmission channel having an average data rate at least equal to the sum of the average data rates supplied by the sources, comprising a packet call input unit (10) connected to the transmission channel, characterized in that:

   - at least some of said sources are cascade mounted, the downstream source (18_n) constituting an interface with the input unit, in that

   - each source (18) comprises means for storing packets and for generating, for each packet, an insertion priority order which is a function of the type of packet, of the number of reserve packets, and of insertion rules associated with the type of packets to be transmitted, and in that

   - each cascade source, with the exception of the most upstream source (18_n+2), has a routing circuit (20) which, on one hand, supplies, at its output, the highest priority among the two priorities applied to its input, and of which one corresponds to the source itself and the second to the upstream of said source and, on the other hand, directs the packet call to the highest priority.
2. Device as claimed in claim 1, characterized in that, the average data rate of the channel being much higher than the average total data rate of the sources and the variations in the available instantaneous data rate only representing a small fraction of the average data rate, each source is arranged to set the order priority of the packet to be transmitted once and for all.
3. Device as claimed in claim 1, characterized in that, the average data rate of the channel being only slightly higher than the sum of the average data rates, the means for generation for at least some of the sources-comprise means which increase the priority of the packets responsive to the waiting time of the packets.
4. Device as claimed in claim 1, 2 or 3, characterized in that each source, other than the upstream source, comprises a routing circuit (20) having a priority comparator (22) controlling a transmission authorization switch (24) and a packet transmission switch (26), the switches of the sources being mounted in cascade in order to cause the sending of packet by that source whose first reserve pocket has the highest priority.
5. Device as claimed in any preceding claim for the insertion of packets to make up the digital multiplex of a television signal of the X MAC/PACKET conditional access type, characterized in that the sound channel sources supplying three digital blocks for four packets are designed to increase the priority thereof as soon as the transmission of a group of three blocks begins.
6. Device as claimed in claim 5, characterized in that each of the sound channel sources is designed so that the priority thereof rises by one for each reserve packet, the increase in priority at the start of the transmission of a group of three blocks being sufficient so that the source that has begun to transmit has a priority higher than that of all the other sound sources attempting to transmit by groups of three blocks in four packets.
7. Device as claimed in claim 4 and 6, for conditional access television signal according to EUROCRYPT standard, characterized in that the entitlement management message sources (EMM), the entitlement checking message sources (ECM) and the zero channel source are assigned fixed priorities, with degrees increasing in that order and lower than those of the sound channels.

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